PURPOSE: To achieve a higher multiplication rate and reduce a dark current upon multiplication by providing an APD chip and optical fibers, and irradiating an optical detection part on the ADP chip with light emitted from the optical fibers in a spot size of a predetermined ratio or more of an optical detection diameter.

CONSTITUTION: A positional relation among single mode optical fibers 1, a rod lens 3, and an APD chip 5 is adjusted such that a spot size of an optical beam 21 is 50% or more of an optical detection diameter on an optical detection part 5a of the ADP chip 5. Such irradiation with the optical beam 21 on the optical detection part 5a of the ADP chip as becoming 50% or more of an optical detection part diameter is because of a reason that optical intensity per unit area at the optical detection part 5a is reduced and a space charge effect is restricted. When the optical beam 21 is out of the optical detection part 5a, a quantum efficiency is reduced and response is deteriorated owing to absorption outside an depletion layer of photons. Accordingly, taking into consideration the reliability of an optical coupling, the upper limit of the spot size of the optical beam 21 is taken to be 90% of the optical detection part.